This tool use HGT-files and build OSM-contourlines.
I use this in a batch file:
hgt2osm --HgtPath=path2newhgt --WriteElevationType=false --FakeDistance=-0.5 --MinVerticePoints=3 --MinBoundingbox=0.00016 --DouglasPeucker=0.12 --OutputOverwrite=true
FOR %%f IN (*.osm.gz) DO call gz2pbf.cmd %%f
For all HGT's in the directory path2newhgt were build new files with names like clN40W006.osm.gz. As next, all files where converted and compressed with gz2pbf.cmd to PBF's.
@set str=%1
@set str=%str:~0,-7%
osmosis --rx %str%.osm.gz --wb %str%.osm.pbf omitmetadata=true
del %1.osm.gz
Just we have for every HGT a new PBF with contoulines for 1°x1°.
For a new map we can take the downloaded map PBF and the necessary contour PBF's and merge the files with osmosis.
osmosis --rbf germany.osm.pbf --rbf clN54E008.osm.pbf --merge ... --write-pbf file=map.osm.pbf omitmetadata=true
This new map.osm.pbf is the source für mkgmap.
Of course, you can prebuild a contour PBF for your country. And you clip this PBF to the country border. But this is another job.